How tricky would this be to machine? I guess a Triac is not up to the job!

[Neale]

Looking at the dimensions you give and trying to scale off the photograph, you are going to leave around 0.8mm wall thickness at the hinge point, or you are going to need to move the axis of the new bore slightly sideways to leave a bit more metal. Is that going to be strong enough for your application? If I were doing this, I might consider a faceplate (if the workpiece is a good shape for clamping) or maybe even look at holding it in a 4-jaw, although I don't know if you could get a good enough hold without marking or distorting the work, especially as the wall thickness reduces. Be interesting to hear how the professionals would tackle it!